X-ray detection is used in a wide variety of fields such as material science, crystallography, spectroscopy, non-destructive testing, and medical devices and applications. For many of these applications, it is advantageous to use X-ray detectors that exhibit a combination of desirable performance characteristics including, for example, high dynamic range, high count rate capability, low noise, high frame rate operation, high resolution, and high detection sensitivity.
State-of-the-art photodetector arrays used for X-ray detection applications typically include a two-dimensional array of pixels, each configured to convert incoming radiation into an analog electric signal (e.g. current, charge or voltage), which is proportional to an incident amount of X-ray radiation. The analog electrical signal can then can be digitized and processed into image data. Therefore, such X-ray detectors generally include a processing unit for obtaining image data from an electrical signal of the photodetector array, and may also include a display device such as a monitor for displaying the image data to a user.
X-ray detectors may be classified according to a method of acquiring X-ray data and a method of converting detected X-rays into an electrical signal. First, current pixel array X-ray detectors may be classified into two types according to their method of X-ray data acquisition: analog integrating detectors, which operate by accumulating charge in an analog storage circuit, and photon-counting detectors, which operate by counting individual X-ray photons using a discrimination circuit. Second, the detection of X-rays itself can be “direct” or “indirect”. Direct X-ray detectors use a semiconductor to directly convert X-ray photons into electric signals in the image sensor. No intermediate steps or additional processes are required to capture and convert incoming X-ray radiation. Indirect X-ray detectors use an X-ray converter such as a scintillator to first convert incoming X-ray radiation into visible light, which is subsequently converted into electric signals in the image sensor.
Analog integrating X-ray detectors can be based on charge-coupled device (CCD) and complementary metal oxide semi-conductor (CMOS) pixel array technologies. An advantage of analog integrating detectors is that they have no intrinsic count rate limitations, and thus can deal with high count rates. However, the integration process accumulates not only the electrical signal generated by the incident photons, but also undesired noise contributions such as read noise and dark current. The presence of these unwanted noise components limits the overall performance of analog integrating detectors in terms of achievable dynamic range and signal-to-noise ratio, especially for weak exposures, where signals tend to fade into the noise, but also for strong exposures, which can saturate the detector.
In contrast to their analog integrating counterparts, photon-counting detectors count individual photons by discriminating logic, and as such are not significantly affected by dark current and readout noise. As a result, high detection sensitivity can be achieved, especially for weak exposures. Yet, photon-counting pixels also have some drawbacks, such as count rate saturation at high incoming photon rates. Also, photon-counting detectors tend to suffer from charge sharing between neighboring pixels, which occurs when incoming photons strike the detector array at pixel boundaries. This can lead to count rate fluctuations as the splitting of photon energy between multiple pixels may prevent any of the pixels from reaching the detection threshold, thus leading to lost counts.